Latest Forecast for Maunakea Observatories
10 AM HST (2000 UTC) Tuesday 06 April 2021

Warnings
None

Cloud Cover and Precipitation Forecast
The summit will remain dry and stable, while scattered high clouds will pass mainly along the southern skies through the night.

Summary of Key Meteorological Variables
Summit temperatures will be near 4 C this afternoon and -1 C for the night. Winds will be from the NW at 10-20 mph for today, increasing to 15-25 mph through the night. Seeing will be near 0.6-0.7 arcseconds, while precipitable water is expected to be in the 1.5-2 mm range for the night.

Discussion
The tradewind inversion will continue to cap low-level moisture near 7-8 thousand feet and ensure a dry/stable summit-level air mass through the next 5 nights. Daytime clouds will be minimal and short-lived throughout the forecast period.

Scattered high clouds will continue to fill in from the NW and eventually north, and will pass through the area over the next 2 nights. There is also a chance that a narrow band of mid-level clouds will slip through the area early tomorrow evening. Regardless, cloud cover should not exceed 30% for another prolong periods over the next 2 nights, while predominately clear skies will prevail for Thursday night. More widespread high clouds are expected to fill in from the west for Friday night and could contribute to extensive cloud cover for much of Saturday.

Precipitable water is expected to increase in the 1.5-2 mm range for tonight, start out near 2 mm for early tomorrow evening, but slip toward 0.7 mm by the end of that night. It may subtly increase to 0.8 mm for Thursday night, then jump to 2-3 mm for Friday night and 3-4 mm for Saturday night.

While there is a chance for calmer skies and better than average seeing for this evening, an increase in low-level/boundary laer turbulence will likely degrade seeing through the night and contribute to poorer than average seeing for much of tomorrow night. Seeing is expected to settle back in near 0.55-0.6 arcseconds as winds subside for Thursday and probably most of Friday night. However, another round of boundary layer turbulence could degrade seeing again for Saturday night.

While a blocking low pattern will persist in the northeastern portion of the Pacific probably through the week, subsidence associated with ridge to the north will prevail in the area during this time. This subsidence will help maintain a well-defined inversion primarily below 7 thousand feet and ensure a dry and stable summit-level air mass throughout the forecast period. Unfortunately, northerly flow between the low and ridge will continue to send patches of high clouds and mid/upper-level moisture into the area mainly over the next 2 nights. A tight wind gradient along the SW flank of the low will also keep summit-level winds near 20 mph, enough to stir up boundary layer turbulence (and perhaps low-level turbulence), which will contribute to poorer than average seeing for most of the next 2 nights. Seeing, PW and CC will improve as the low retracts a bit and shifts subtly eastward for Thursday night. However, winds may pick up and deteriorate seeing again as the low shifts westward and rebuilds over the weekend. There is also a good chance this movement will strengthen a rather broad sub-tropical jet overhead, which could gain access to broken high clouds flowing out of the tropics and send it overhead during that time.